The present invention concerns the operation of electrolytic cells of type Hall-Hxc3xa9roult. More precisely, the present Invention relates to improved control of process gases and limitation of emissions of process gases into the hall atmosphere during maintenance on the cells.
The upper part of an electrolytic cell of type Hall-Hxc3xa9roult usually comprises an anode superstructure consisting of covers or an enclosure including a gas extractions system. Some auxiliary equipment may also be attached such as an anode beam with jacks, crust breakers and a system for raw material dosing inside the superstructure. In order to make the anode superstructure as gastight as possible, covers are constructed which are designed to separate the hall atmosphere from the process gases. The task of the covers is to preserve/increase the underpressure inside the anode superstructure in connection with a given gas extraction. This will lead to the process gases being transported into the anode superstructure and on out to collection channels to a greater or lesser extent, depending on the design of the system. At a constant extraction quantity, the system must be dimensioned so that opening the covers entails a limited emission of process gases to the hail atmosphere in order that the air of the working environment is not of too poor quality.
GB 2158226 discloses extraction of gases from an electrolysis cell, where the cell""s superstructure is provided with a plurality of evacuating means. Said means are arranged successively in the longitudinal direction of the superstructure, and each of them have provisions for controlling the flow of the extracted gas and to registrate the temperature of same. The opening of a cell cover will activate an increased suction extraction mode in the actual evacuating means in response to a drop in the temperature of the extracted gas passing therethrough. This solution is thus dependent upon the occurrence and registration of cold gas in the duct of at least one of the evacuating means to increase the extraction of gases from the cell. One problem with this solution is that the cold gas that enters the cell through an opened cover, will tend to xe2x80x9cfallxe2x80x9d down towards the crust of the cell and be heated up on Its way towards the evacuating means. Followingly there may be a severe delay in the activation of the increased suction depending upon the internal gas flow patterns in the superstructure of the cell. This will subsequently represent a risk of outflow of process gases from the cell to its surroundings. Further, as this principle of extraction is based on activation/deactivation of increased suction in relation to measured temperature at plural points within the superstructure, extraction may be activated at non-optimal locations initiating turbulent flow patterns allowing process gases to escape from the uncovered opening.
WO 95/22640 relates to an arrangement for closing and cooling the top of an anode casing for a Sxc3x8derberganode. The top of the anode casing is equipped with at least one cover having openings for the contact bolts and at least one off-gas opening. The amount of gas removed from the top of the anode through the off-gas opening is controlled in such a way that a sufficient diminished pressure is provided in the top of the anode that surrounding air will flow through air gaps arranged between the cover and each of the contact boltse. This flow will be in such an amount that gas from the top of the anode does not escape through the air gaps and to keep the temperature of the top of the anode calling below a preset temperature. It is further disclosed that the anode top may include plural side covers, but it is not indicated any increased suction extraction when the covers are opened.
The present invention concerns improvements in the extraction of process gases from electrolytic cells, whereby the disadvantages of the prior art can be avoided. Moreover, the present invention comprises improvements in the flow pattern of the gas inside the anode superstructure, which means that it is possible to achieve improved extraction conditions even with a reduced extraction quantity.